The typical landing gear for a helicopter comprises crosstube members rigidly attached to the base of the helicopter fuselage and joined at the ends to horizontally disposed skids. The crosstubes and skids are made of a flexible tubular material to support the aircraft and perform as a shock member upon ground contact.
The flexibility of the landing gear structure, however, causes the helicopter airframe and landing gear together to have various vibrational modes. When the frequencies of any of these vibrations are the same or are related to the vibrational frequency of the helicopter rotor blade about its lead-lag hinge, energy is transmitted from the rotating system to the fixed (nonrotating) system. Under these conditions, a potentially serious problem termed "ground resonance" arises. When ground resonance occurs, the vibration of the rotor blade causes the helicopter fuselage to vibrate, the vibrational energy of the rotor blade being transferred to the fuselage with the resulting vibrations being reinforced. The reinforcement of the vibrations causes violent shaking of the aircraft which can often lead to structural damage.
Therefore, a need exists for a landing gear structure which tunes the helicopter airframe to have a natural frequency that is lower than that of the rotor blades when operating at or below their normal rotational speed. The helicopter airframe natural frequency must be offset from that of the rotating rotor blade to prevent the occurrence of ground resonance.